1. Field of the Invention
The present invention relates to a beam shaping and beam splitting device for converting a cross sectional shape of an incident light beam and splitting a return light beam from the incident light beam, said return light beam being obtained after reflecting the light beam by a reflection surface. The present invention also relates to an optical head comprising a beam shaping and beam splitting device for use in an information recording and/or reproducing apparatus.
2. Description of the Related Art
There has been proposed a technique for effecting both beam shaping and beam splitting. For instance, Japanese Patent Publication No. 3-21973 discloses an optical head for use in an optical disk recording and/or reproducing apparatus comprising a laser light source, in which a single prism is used for correcting an elliptical cross section of an incident laser light beam into a circular cross section as well as for splitting a return laser beam reflected by an optical disk from the incident laser light beam.
FIG. 1 is a schematic view showing the above. mentioned known optical head. In this optical head, between a semiconductor laser 21 and a record medium 23 of an optical disk 22, there is arranged a prism 24, and between the semiconductor laser 21 and the prism 24 there is arranged a collimator lens 25. An objective lens 26 is arranged between the prism 24 and the record medium layer 23.
The prism 24 has a special coating on its incident surface 24a for splitting the incident laser beam from the return laser beam from each other. Further, the incident surface 24a is arranged with respect to the incident laser beam such that an elliptical cross sectional shape of the incident laser beam emitted from the semiconductor laser 21 and being incident upon the prism 24 is converted into a circular cross sectional shape. The laser beam propagates within the prism 24 and emanates from a second surface 24b of the prism 24 perpendicularly thereto. Then the laser beam is projected onto the optical disk 22 by means of the objective lens 26 and is reflected by the optical disk. In the specification of the present application, this laser beam is called a return laser beam. Then, the return laser beam is made incident upon the second surface 24b of the prism 24 and propagates within the prism toward the first surface 24a. The return laser beam is reflected by the coating on the first surface 24a toward a third surface 24c and emanates from the third surface 24c perpendicularly thereto. That is to say, the return laser beam can be split from the incident laser beam.
The laser beam emanating from the prism 24 through the third surface 24c is received by a light receiving element 28 by means of a lens 27. The lens 27 and light receiving element 28 constitute a photodetecting system for receiving the return laser beam reflected by the record medium layer 23 of the optical disk 22.
The laser beam emitted by the semiconductor laser 21 generally has an elliptical cross sectional shape. This laser beam is made incident upon the incident surface 24a of the prism 24 via the collimator lens 25 at such an incident angle .alpha. that the elliptical cross sectional shape of the incident laser beam is converted into a circular cross sectional shape. Then, the laser beam emanates from the prism 24 via the second surface 24b and is made incident upon the record medium layer 23 of the optical disk 22 by means of the objective lens 26.
The return laser beam reflected by the record medium layer 23 of the optical disk 22 is made incident upon the second surface 24b of the prism 24 by means of the objective lens 26. Within the prism 24, the return laser beam is reflected by the special coating applied on the surface 24a and emanates from the prism via the third surface 24c. The laser beam is finally made incident upon the photodetector 28 by means of the lens 27. In the manner explained above, the prism 24 serves as the beam shaper as well as the beam splitter.
The known optical head explained above has an advantage that the beam shaping means and beam splitting means are constituted by the single prism, so that the number of parts of the optical head can be reduced and thus the optical head can be made small in size. However, a beam separation angle .beta. between the incident laser beam and the return laser beam is relatively large. This is due to the fact that the incident angle .alpha. of the incident laser beam with respect to the first surface 24a has to be set to achieve a desired beam shaping and the laser beam reflected by the record medium layer 23 of the optical disk 22 has to be reflected by the first surface 24a. Therefore, the laser light source 21 and photodetector 28 have to be arranged to be separated from each other, and therefore the optical head could not be made sufficiently small.